Recently, with the tendency toward down-sizing of various electronic devices and with the popularization of IC cards, electronic parts such as semiconductor wafers and others are desired to be thinned further more. Accordingly, semiconductor wafers heretofore having a thickness of around 350 μm are needed to be thinned more to have a thickness of at most around 50 μm or so. For enhancement of producibility, further increasing the diameter of semiconductor wafers is now under investigation.
In general, in production of semiconductor wafers, a circuit pattern is formed on the front surface of a wafer, and then the back surface of the wafer is ground with a grinder or the like so as to have a predetermined thickness. In this stage, for the purpose of protecting the front surface of the wafer, in general, the back surface of the wafer is ground while a pressure-sensitive adhesive sheet is stuck to the front surface of the wafer. Further, after the wafer is processed to be thin, the wafer may be transferred to the next step while a pressure-sensitive adhesive sheet is kept stuck to the front surface of the wafer.
However, in case where the back surface of the wafer is ground to an ultrathin level while the front surface of the wafer is protected with a pressure-sensitive adhesive sheet, the ground wafer may be often warped. The warped wafer has a problem in that it may be often cracked during transportation thereof or in peeling the pressure-sensitive adhesive sheet therefrom. In particular, in grinding large-size wafers having a diameter of 8 inches or 12 inches that are often used in the art or in grinding thin wafers for IC cards or the like, the problem of warpage is serious.
It is considered that the warpage of wafers after grinding may be greatly influenced by the residual stress remaining in the pressure-sensitive adhesive sheet stuck to wafers. When a wafer with a pressure-sensitive adhesive sheet stuck thereto is ground ultrathinly, it is considered that the residual stress may be higher than the strength of the wafer and the wafer may be warped due to the force to remove the residual stress. Accordingly, for reducing the residual stress, the constitution of the pressure-sensitive adhesive sheet has been modified or improved variously, and a constitution not yielding a residual stress has been proposed. For example, Patent Reference 1 proposes a pressure-sensitive adhesive sheet for protection of semiconductor wafer, which includes a substrate film and a pressure-sensitive adhesive layer and in which the modulus of tensile elasticity of the substrate film is 0.6 GPa.
Patent Reference 2 proposes a pressure-sensitive adhesive sheet for processing semiconductor wafer, which includes a substrate and a pressure-sensitive adhesive layer formed on the substrate, and in which the stress-relaxation rate of the pressure-sensitive adhesive sheet after 1 minute at an elongation of 10% in the tensile test thereof is at least 40%.
However, the properties of these pressure-sensitive adhesive sheets are not always the best for preventing ground wafers from being warped in ultrathinly grinding semiconductor wafers or in grinding large-diameter wafers. Accordingly, it has been desired to provide a pressure-sensitive adhesive sheet for protection of semiconductor wafer which can more effectively prevent ground wafers from being warped.
With the recent tendency toward ultrathinly grinding wafers in the art, it has been also desired to prevent wafers from being cracked by the stress in grinding them and to prevent the edges of wafers from being broken off.
Further with the recent tendency toward high-density, down-sized, high-function semiconductor packages, chip stacking technology is promoted, in which the thickness fluctuation between ultrathinned wafer chips is considered problematic. The thickness fluctuation between chips is because the thickness fluctuation in the pressure-sensitive adhesive sheet to protect patterns in wafer grinding is directly transferred as such to the ground chips, and therefore the accuracy of the thickness of the pressure-sensitive adhesive sheet has been an important issue in the art.
Patent Reference 1: JP-A-2000-212524
Patent Reference 2: JP-A-2000-150432